1. Field of the Invention
The present invention relates generally to power-assist devices. More particularly, the present invention relates to a method and system for dynamically controlling a power-assisted lift system to continuously reduce operator strain in a real-time mode.
2. Discussion of the Related Art
In the automotive industry, lift devices are often employed in car assembly line stations to assist human operators with difficult tasks. These devices are most useful in stations requiring the lifting and manipulation of heavy loads. A typical device is primarily designed to balance the gravity of a load during lifting and travel around an assembly line station. The human operator, however, must still push or pull the device in order to move it horizontally for parts assembling. These actions require the operator to either accelerate or decelerate the load-carrying device each time a change in direction is desired. This directional change is particularly difficult when each major link of the device is large in mass and has significant moments of inertia which add to the amount of work to be done. To further aggravate the problem, a typical operation in a car assembly line will often be repeated in excess of 50 times per shift. This repetition has the potential to cause cumulative wrist or arm injury after consecutive months of work. Power-assisted lift devices were therefore developed to address the major concerns of ergonomics and human factors engineering.
Typical power-assisted approaches provide lift devices with four-axis motion. These devices are driven by servo-motors and guided by a closed-loop feedback of force data. In one system manufactured by FANUC Robotics, Inc., the force data are monitored and measured by a six-axis force sensor mounted behind the manual handle of the device. The current status of the feedback loop, however, is based only on the kinematics/statics relation between Cartesian positions/forces and joint positions/torques of the device. Thus, these systems have a noticeably slow response to operator-induced changes in direction. The slow response results in significant strain on operators any time a change in direction is attempted. It is therefore desirable to use joint data to provide a dynamic compensation within a substantially shorter response time.